Bipolar circuits usually have sufficient power to drive data buses directly. The output capability of bipolar transistors provide for large current pulses at the operating voltage.
Metallic oxide and silicon-on-sapphire circuits use small transistor configurations for the on-board logic functions but the output stages are usually required to be large so as to have sufficient drive capability for peripheral devices and buses. The large output stages are expensive and use considerably large areas of the substrate. They are usually high impedance devices so that their switching speed is limited by the external capacitance that they must drive. Complicated precharge circuits are used to increase the switching speed of these high impedance switches but such circuits require additional timing and logic which make the precharge approach to speed-up undesirable.
The output drive capability of integrated circuits such as microprocessors are usually a design compromise between the worst case condition and the available area for structuring the output stages. Sometimes in practice, the entire available drive capability is not always required so the extra drive capacity is wasted.
If a microprocessor is designed having the minimum possible output drive capability, the result is a less expensive and smaller microprocessor. It would, however, require external drive circuits, increasing the on-board components in most applications. This is not usually a disadvantage in large systems using many on-board devices anyway.
The external driver circuits to be used would preferably be tri-state. That is, when activated, the output circuit assumes one of two logic states to drive the connected devices. These states provide low impedance paths to ground or to the power supply. When inactive, however, the driver presents a high impedance which does not load down other activated drivers on the line. This high impedance state is the third state.
U.S. Pat. No. 3,912,947 shows a sequencing circuit for a tri-state output circuit. The purpose of the disclosed circuit in the patent is to provide the activated output signal for a time period that is longer than the gating circuit. The circuit is unidirectional, however, and does not provide the directional-enabling signal sequencing that is the subject matter of this application.